This invention relates to inverters for electroluminescent (EL) lamps and, in particular, to an inverter producing lower amplitude current spikes through an EL lamp having one electrode grounded.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder and a dielectric layer between two conductive electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and current through the EL lamp ceases. The phosphor particles radiate light in the presence of a strong electric field, using relatively little current. As used herein, an EL “panel” is a single sheet including one or more luminous areas, wherein each luminous area is an EL “lamp.”
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
An inverter for EL lamps is typically what is known as a “flyback” inverter in which the energy stored in an inductor is supplied to the EL lamp as a small current at high voltage. If one considers a system as including a battery, an inductor, and an EL lamp, the prior art discloses switching one of these elements to obtain an alternating current through the lamp.
FIG. 1 is a schematic diagram based upon U.S. Pat. No. 4,527,096 (Kindlmann), in which the EL lamp is switched. When transistor 14 turns on, current flows through inductor 15, storing energy in the magnetic field generated by the inductor. When transistor 14 shuts off, the magnetic field collapses at a rate determined by the turn-off characteristics of transistor 14. The voltage across inductor 15 is proportional to the rate at which the field collapses. Thus, a low voltage and large current is converted into a high voltage at a small current.
The current pulses are coupled through diode 16 to the DC diagonal of a switching bridge having EL lamp 12 connected across the AC diagonal. The transistors in opposite legs of the bridge conduct alternately to reverse the connections to lamp 12. The bridge transistors switch at a lower frequency than transistor 14. The four bridge transistors are high voltage components, adding considerably to the size and cost of the circuit. The circuit is not single ended, i.e. one cannot ground one side of lamp 12. Inductor 15 discharges directly into lamp 12, producing current spikes. The bridge circuit disclosed in the Kindlemann patent is also sometimes referred to as an H-bridge output, where the switching transistors form the posts and the EL lamp forms the cross-bar of the H.
U.S. Pat. No. 5,436,283 (Sanderson) discloses a variation of the circuit shown in FIG. 1. The variation includes a storage capacitor connected across the DC diagonal of the bridge and a constant current source in each of the two upper legs of the bridge. This reduces current spikes but does not provide a single ended output. U.S. Pat. No. 5,686,797 (Sanderson) includes the same disclosure as the '283 patent.
FIG. 2 is a diagram taken from U.S. Pat. No. 5,313,141 (Kimball). U.S. Pat. No. 5,668,703 (Rossi et al.) discloses substantially the same circuit, in which the inductor is switched to obtain an alternating current. Inverter 20 is a three terminal device having supply terminal 21, ground terminal 22, and high voltage terminal 23. Within inverter 20, first switching circuit 25 pumps current pulses through inductor 26 and second switching circuit 27 connects current pulses from inductor 26 to EL lamp 12 through high voltage terminal 23.
Switching circuit 25 includes switches 31 and 32 forming a series circuit with inductor 26 between supply terminal 21 and ground terminal 22. Switching circuit 27 includes switches 33 and 34 connected between each end of inductor 26 and high voltage terminal 23. Specifically switch 33 is connected between end 37 of inductor 26 and high voltage terminal 23. Switch 34 is connected between end 38 of inductor 26 and high voltage terminal 23.
When switches 31 and 34 are closed (conducting) and switch 33 is open (non-conducting), switch 32 opens and closes at a high frequency, producing a series of high voltage pulses that are connected from terminal 38 of inductor 26 through switch 34 to high voltage terminal 23. When switch 32 opens, the field on inductor 26 collapses, attempting to maintain the current flowing in the same direction as before switch 32 opened. The only current path remaining is through switch 34 to lamp 12, charging the upper electrode of lamp 12 positively. Diode 35 blocks current from lamp 12 to ground when switch 32 is closed.
For the second half of the cycle, switch 32 closes and remains closed, switch 34 opens and remains opened, and switch 33 closes and remains closed. Switch 31 opens and closes at high frequency, producing a series of current pulses through inductor 26. During this half of the cycle, terminal 37 of inductor 36 is connected through switch 33 to lamp 12. When switch 31 opens, the collapsing field in inductor 26 tries to maintain the current flowing in the same direction as before switch 31 opened. Since terminal 37 is connected to lamp 12, this current is drawn from lamp 12, discharging the upper electrode of lamp 12 and eventually charging the upper electrode negatively. Diode 36 blocks current from lamp 12 to supply terminal 21 when switch 31 is closed. After a given number of high frequency pulses, the upper electrode of lamp 12 is at a peak negative voltage and the cycle ends.
FIG. 3 is a functional diagram of a circuit based upon U.S. Pat. No. 5,854,539 (Pace et al.), in which the battery is switched to obtain an alternating current. The circuit operates similarly to the circuit of FIG. 1, except that the battery connections are periodically reversed instead of the lamp connections. Inductor 41 dumps current directly into EL lamp 12, producing undesirable current spikes. Like the circuit shown in FIG. 1, one terminal of lamp 12 cannot be grounded.
An inverter having a single ended output has several advantages over inverters with bridge type outputs and is very much desired in the market. Unfortunately, the advantages come with a trade-off, viz. an inductor discharges directly into an EL lamp, producing a current spike that causes excessive power consumption, reduced efficiency, and difficulty driving some high impedance EL lamps having an area greater than approximately 15 cm2. Specifically, some thin, screen printed EL lamps adapted for backlighting keypads exhibit high impedance. Other lamps do as well, depending upon materials and thicknesses. EL lamps have a nominal capacitance of 0.47 nf per square centimeter. Discharging an inductor directly into a capacitor greater than about 10 nf can cause significant current spikes. Semiconductor components used for implementing an inverter must withstand not only the high voltage from an inductor but the current spike as well. This increases the cost of implementing the inverter as an integrated circuit and restricts the technologies that can be used for making the inverter.
In view of the foregoing, it is therefore an object of the invention to provide an inverter having a single ended output and reduced current spikes.
Another object of the invention is to provide a single ended inverter that can be implemented in bipolar or CMOS technologies.
A further object of the invention is to improve the efficiency of an inverter having an single ended output.